The present invention generally relates to an electrical switch which includes a fluid level sensor, and more particularly to a switch that is adapted to automatically complete an electrical circuit when the volume of fluid being monitored is below a predetermined level.
In many devices that house fluids within a reservoir, it is desirable to monitor the fluid level. In certain devices it may be advantageous to change an operating parameter when the volume of fluid falls below a predetermined level. For instance, it would be desirable to automatically shut off an internal combustion engine if the level of lubricating oil in the engine crankcase becomes inadequate. It also would be desirable to shut off a pump such as that used in a bilge of a boat, once the water level is sufficiently reduced. Likewise, it would be desirable to provide a low fluid level warning to users of consumable fluid products stored in tanks, such as home heating oil.
Problems associated with ignoring low fluid conditions vary from matters of inconvenience, to safety, to major system damage. For instance, the result of a loss of oil or a low oil condition during the operation of relatively small spark ignition engines, such as those used in power generators or lawn mowers, are well known in the art and are described, in part, in U.S. Pat. No. 4,600,820. In general, if such engines experience operating conditions involving a rapid loss of oil, unintended extreme tilting of the engine, or simply insufficient maintenance, they may lack access to a sufficient supply of oil to adequately lubricate the internal components of the engine. Continued operation of an engine experiencing such a condition may lead to severe engine damage and could present a safety hazard.
The switch used with spark ignition engines that is disclosed in the aforementioned U.S. patent, exhibits certain disadvantages that are overcome by the present invention. In particular, the prior art device is designed to shut off an engine by grounding the ignition system in the event of a low oil level. The switch is mounted horizontally, through a side wall of an engine crankcase. Hence, the device is likely to be overly sensitive to engine tilting because the device will likely be mounted closely adjacent a side wall of the crankcase. Also, the entire switch assembly, including its balance beam float assembly and switching lever, are contained within a horizontal encapsulating shield having slots and holes. The slots and holes permit communication with the oil in the crankcase and the escape of foam and bubbles in the oil.
The horizontal mounting of the prior art assembly in an engine crankcase, and its need to be located in the vicinity of the top surface of the oil when the oil level is at a predetermined maximum recommended level, also subject the device to considerable turbulence and fluctuations in the oil due to splashing and aeration generated by the use of a common crankshaft mounted splasher device. These conditions may result in less reliable performance of the switch, and may lead to intermittent operation of the ignition system. In addition, the balance beam assembly utilized in the prior art device does not appear well suited to cope with more violent and higher amplitude vibrations, such as those encountered once the ignition has been interrupted in a spark ignition engine. Similar disadvantages would be experienced if the prior art device is used to automatically shut off a diesel engine by interrupting an electronically controlled fuel supply system under low oil level conditions.
Additional prior art designs used for low fluid level conditions, such as in automatic engine shut off applications, include a glass reed switch with a floating magnet. One problem with this technology is that the pair of glass reeds in such a switch are very thin and tend to become welded together if too much electrical current is applied to the switch. In the case of an ignition cut off switch which is designed to ground the ignition system of a relatively small spark ignition engine under low oil level conditions, the reeds may become permanently joined due to a current that may be in a range as low as approximately 2 to 5 amps. In the event of such a switch failure, even after adding oil to reach a safe level, the ignition system will remain grounded and the engine will not be able to be restarted until the switch is replaced.
Some manufacturers using glass reed switches have added electrical devices such as a Triac to accommodate the operating amperages. Such additional components add significantly to the cost of the switch, yet do not alleviate other problems with glass reed switches, such as sensitivity to high temperature and shock.
In light of the shortcomings of the above-mentioned exemplary prior art devices, it is desirable to have a switch which combines fluid level sensing and automatic opening and closing of an electrical circuit, and which permits an improved mounting position to reduce sensitivity to fluid reservoir tilting. It is further advantageous to provide a switch device that is less sensitive to turbulence and fluctuations in fluid level that are present in some operating environments, such as splashing and aeration. It also is advantageous that a fluid level sensing switch have rotational torque characteristics that are less susceptible to intermittent operation during large amplitude, varied and lower frequency movements, such as the vibrations commonly experienced during shut down of an internal combustion engine. The present invention overcomes the disadvantages of the prior art, while providing the above mentioned desirable features of a fluid level sensing switch.
The present invention is generally embodied in an improved fluid level sensing switch. The switch comprises a base, a float pivotally connected to the base, a counterweight pivotally connected to the base, a linkage connecting the float and the counterweight, a conductive damper, an electrical terminal and a conductive pin.
When the volume of fluid is at or above a predetermined sufficient level, the switch remains open. If the volume of fluid falls below the predetermined sufficient level, then the float pivots downward, forcing the counterweight into the conductive damper, which in turn contacts the terminal. In this condition, the switch is closed and completes the circuit.
In another aspect of the invention, the counterweight is formed of a material that is of greater density than the material of the float.
In still another aspect of the invention, the conductive damper, conductive pin and terminal may be of sufficient physical size to enable them to carry sufficient amperage reliably, so as not to require additional electronic devices, such as a Triac, to accommodate the operating amperage in the circuit. Indeed, the invention lends itself to use of components tailored to specific needs, and can be structured to carry relatively low amperages, or current in excess of 60 amps.
In a further aspect of the invention, the electrical terminal includes a magnet. The magnet provides a hysteresis effect to prevent unintended intermittent opening and closing of the switch, such as during the more violent vibrations encountered in shutting off an engine. The magnet helps maintain contact between the conductive damper and the terminal, thus keeping the switch closed, for instance to ground an engine ignition system, until a sufficient fluid level raises the float. As the float is raised by an increase in fluid level, the linked counterweight pivots which, in turn, forces the conductive damper to break its magnetic coupling to the terminal, opening the switch.
In another aspect of the invention, the switch. has a cover attached to the base.
In still another aspect of the invention, the cover is adapted to be snap fit over the base.
In a further aspect of the invention, the cover has at least one opening arranged at or near its top, and at least one opening, such as a hole or slot near its bottom. The upper opening or openings allow air to pass out of the fluid that is within the cover, while the lower opening or openings permit fluid near the bottom of the reservoir in which the switch is housed to enter and exit the device through the cover. Communication with fluid that is very low in the reservoir reduces the sensitivity of the switch to conditions that involve turbulence, splashing or aeration, because the switch is protected by the cover, and because fluid near the bottom of the reservoir is less likely to be turbulent or to contain air bubbles or foam.
In a further aspect of the invention, the base of the switch is adapted to be mounted vertically, to extend upward from the bottom of a reservoir. Therefore, sensitivity to tilting of a reservoir can be reduced by mounting the switch very close to the center of the reservoir, where the fluid level will generally be most stable. Moreover, in a particularly advantageous aspect of the invention, the switch may be adapted to be mounted through the bottom wall of a reservoir. This mounting configuration would permit convenient withdrawal of the switch from the bottom of the reservoir for service or replacement.
In yet a further aspect of the invention, the switch uses a four bar linkage to generate sufficient torque to operate the float and counterweight, and to overcome wet film sticking between components. The four bar linkage further permits a very compact switch structure. The switch enjoys damping a wide range of vibration amplitudes and frequencies due to the enhanced torque management characteristics obtained by use of a float and a counterweight that are each separately pivotally mounted and that are connected via a linkage, in further conjunction with a low mass conductive damper.
It is to be understood that both the foregoing general description and the following detailed description of a preferred embodiment of the present invention in the form of a low oil level ignition cut off switch are exemplary, provided for purposes of explanation only, and are not restrictive of the invention as claimed.